1. Field
This application relates generally to distance ranging, and to distance ranging verification.
2. Background
Distance ranging involves determining a distance between two locations. In a typical scenario a device such as a rangefinder measures a distance from the rangefinder to another object. A distance ranging device may employ a variety of technologies such as laser, radar, sonar, and various forms of radio-frequency (“RF”) signaling. For convenience, the term distance ranging will be referred to herein simply as ranging.
Ranging may be employed in a communication system through the use of RF signaling. For example, in a wireless communication system a ranging device may determine the amount of time it takes for a signal to travel from the ranging device to another device, the amount of time it takes for a signal to travel from the other device to the ranging device, or both. The ranging device may then calculate the distance between the devices based on any of these times and the known propagation speed of the RF signals (e.g., the speed of light).
Some systems employ a two-way message exchange mechanism to measure the relative distance between two devices. For example, a first device may send a ranging packet to second device. The second device may then send a reply packet back to the first device whereby the reply packet indicates the amount of time it took the second device to transmit the reply packet after receiving the ranging packet (i.e., second device's turnaround time). The first device may then use this turnaround time to determine the actual propagation time of the packets. Here, the first device calculates the total round-trip time as the amount of time that elapsed from the time it transmitted the ranging packet to the time it received the reply packet. The first device may then determine the actual propagation time by subtracting the turnaround time from the total round-trip time.
In practice, this form of two-way message exchange ranging is susceptible to being compromised by the second device. For example, the second device may send false information to the first device in the reply packet to make it appear to the first device that the second device is closer than it actually is. For example, the second device may report that it received the ranging packet from the first device earlier than it actually did and also report that it transmitted the reply packet at a later time than it actually did.
One approach for addressing this problem is to authenticate the second device to the first device using a public/private key exchange and an authentication server. Once authenticated, the first device may then trust the information reported by the second device. In practice, however, ranging authentication also may be needed for devices that can not be trusted in this manner. Accordingly, a need exists for more reliable ranging techniques.